The present invention relates generally to building automation systems. More particularly, the invention relates to an event-driven or asynchronous object-oriented control system that is able to provide support for synchronous object execution with respect to real time, including those used to maintain closed-loop control over a system.
Many of the control systems within a building automation environment employ closed-loop control. A feedback signal is derived from the output of a controller or device and that signal is compared with the input signal to calculate an error signal. The error signal is then used to change the setting of the device or controller, causing the output to more closely match the desired input.
Originally, control systems were constructed using analog components, in which case, the input and output signals and the error signal were continuous with respect to time. The voltage level at any point in time thus represented the signal value. According to well-known control theory the performance of these analog systems could be modified and improved by altering the feedback signal in a time-dependent manner. For example, one common closed-loop control architecture uses proportional integral derivative (PID) control to improve closed-loop performance. In the classic PID control, the system under control is mediated by three components: a proportional component, an integral component and a derivative component. The proportional component is obtained by multiplying the feedback signal by a predetermined constant. The integral component is obtained by integrating the feedback signal with respect to time. The derivative component is determined by taking the derivative of the feedback signal with respect to time.
When digital control systems were introduced, they were originally designed to mimic the behavior of the earlier analog systems. Thus digital techniques were developed to periodically sample the output of devices and controllers, so that the sampled output could be used as a feedback signal for closed-loop control. Various control algorithms, such as the PID control algorithm, were then adapted for digital control by using the sampling rate of the digital signal as the time base for performing derivatives and integrals. Thus the PID control algorithm is by its nature synchronous with respect to a predetermined clock.
Because many control processes require some form of synchronicity with respect to time, conventional building automation systems are engineered to guarantee that all synchronous operations are performed within certain allotted time frames. Typically this is done by defining an operating cycle and by then assigning different portions of that cycle to different control processes. In this way the entire system is engineered to be synchronous with the predefined cycle. In essence, the control algorithm is merely performing a repeating loop that is synchronized to execute fully from start to finish within the predefined cycle.
While synchronous control software can be used advantageously in smaller systems, the requirement of imposing synchronicity becomes a deterrent when complex distributed systems are involved. But for the need to support synchronous operation, complex building automation systems might be better implemented using event-driven asynchronous technology. However, heretofore the asynchronous model has not worked well in building automation applications, because many of the subsystems (such as PID controllers) require synchronous execution.
In a truly asynchronous operating environment there is no guarantee that a given process will run from start to end within a given time. In turn, a subsequent process may not begin to run at its schedule time. Thus, the second process cannot be guaranteed to run from start to end within a predefined time interval and this has traditionally ruled out the inclusion of a deterministic or synchronous with respect to real-time control process within an asynchronous operating environment.
To address this problem, the invention provides a distributed-object building automation system that provides full support for synchronous object execution. Specifically, the invention provides real time support for standard control objects that automatically alters their process and/or its data based on a measurement of elapsed time. The process thus yields the results that would be obtained through synchronous execution, even though the process may not have been performed in synchronism with a defined time interval.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.